1. Field of the Invention
The present invention relates to a thermosensitive recording paper, and more particularly to a thermosensitive recording paper free from a curling problem even when it is stored in the form of a small roll, which can be properly stacked in a paper tray after thermal recording, and is capable of yielding clear images with high image density by application of a small amount of thermal energy.
2. Discussion of the Background
Various recording materials are conventionally proposed, which utilize a coloring reaction between a colorless or light-colored leuco dye and a color developer under application of heat or pressure.
A thermosensitive recording material, one of the conventional recording materials, has the advantages that it does not necessitate the complicated processes of development and image-fixing, thermal printing can be accomplished in a short time with a relatively simple apparatus, the generation of noise is significantly small in the thermal printing, and the manufacturing cost is low. Because of the aforementioned advantages, the thermosensitive recording material is useful as a recording material for use in electronic computers, facsimile apparatus, ticket vending apparatus and label recorders.
In recent years, as the demand for thermal recording is increasing, a thermal recording apparatus capable of yielding images at a high speed is demanded. As a result, the development of a thermosensitive recording paper which can cope with a high-speed thermal recording apparatus is also desired.
To cope with the high-speed thermal recording, there is a demand for a thermosensitive recording paper which can be brought in close contact with a thermal head of a thermal recording apparatus, thereby improving the efficiency of the heat conduction from the thermal head to the thermosensitive recording paper.
In order to meet the above demand, various methods have been studied and proposed for increasing the surface smoothness of a thermosensitive coloring layer of a thermosensitive recording paper.
For example, Japanese Laid-Open patent application 54-115255 describes that a thermosensitive recording material comprising a thermosensitive coloring layer with a smoothness of 200 to 1000 seconds in terms of Bekk's smoothness can cope only with heat pulses of about 5 to 6 msec, and therefore it is necessary to subject the thermosensitive coloring layer to a surface treatment to obtain a smoothness of 1100 seconds or more in order to meet heat pulses of 1 msec or less for high-speed thermal recording.
However, when the thermosensitive coloring layer of the thermosensitive recording material is subjected to a surface treatment until the Bekk's smoothness thereof attains to 1100 seconds or more, colored fogging takes places in the recording material because of the pressure applied thereto in the course of the surface treatment. To solve the problem of the fogging, the aforementioned application proposes to subject a support sheet for the thermosensitive recording material to a surface treatment to obtain a Bekk's smoothness of 500 seconds or more before the formation of a thermosensitive coloring layer thereon.
However, even though the support sheet of the recording material is subjected to calendering to increase the Bekk's smoothness, when a coating liquid is coated on the support sheet to form a thermosensitive coloring layer thereon, the fibers contained in the support sheet are swollen while in contact with the water contained in the coating liquid, so that the smoothness of the support sheet is decreased. The result is that the surface smoothness of the thermosensitive recording material cannot be substantially improved by the above method.
Japanese Patent Publication 52-20142 discloses a surface treatment method of increasing the Bekk's smoothness of a thermosensitive coloring layer of a thermosensitive recording material up to 200 to 1000 seconds by calendering. This method, however, readily causes a fogging in the thermosensitive coloring layer in the course of the calendering. Furthermore, the non-uniformity of the basis weight of the recording material is intensified and thus it is difficult to bring the recording material into close contact with a thermal head of the thermal recording apparatus. The result is that the image density obtained is decreased.
In Japanese Laid-Open patent application 62-25084, in order to obtain the desired surface smoothness of a thermosensitive recording material, the distribution of the length of the pulp fibers contained in a support sheet of a thermosensitive recording material is specified in such a manner that the residual amount of the pulp sifting through a 24-mesh screen is 10 wt. % or less and the total amount of the residual pulp sifting through the 24-mesh screen and the residual pulp shifting a 42-mesh screen is 60 wt. % or less of the absolute dry weight of the pulp for the support sheet in accordance with a sieve analysis in Japanese Industrial Standards (JIS)-P-8207. The distribution of the fiber length of the pulp contained in the support sheet is thus defined, so that the close contact properties of the thermosensitive recording material to a thermal head can be improved. However, the distribution of the fiber length varies depending on the kind of pulp, so that the distribution of the fiber length in a support sheet cannot necessarily be determined by this method. Therefore, high surface smoothness and uniform texture cannot always be obtained in the support sheet by the above method.
The important requirements for the thermosensitive recording paper includes not only the above-mentioned high thermosensitivity, but also the stacking property of the thermosensitive recording material, which is defined by the property that the thermosensitive recording material cut from a small thermosensitive recording material roll is neatly stacked on a paper tray after being subjected to thermal printing.
To carry out the thermal printing, a small roll of the thermosensitive recording paper is incorporated into a thermal recording apparatus such as a facsimile apparatus. The recording paper is then subjected to the thermal printing as reeled out of the small roll thereof, cut into a predetermined size with a cutter mounted in the apparatus, and then discharged onto a paper tray.
The aforementioned small roll of the thermosensitive recording paper is generally prepared by winding a recording paper having a length of 50 to 100 m around a paper core with a diameter of 30 to 40 mm. The longer the storage time, the more easily the recording paper tends to curl. In addition, the thermosensitive recording paper in the vicinity of the paper core often has a severe curling problem. The curled recording paper causes paper jamming in the thermal printing apparatus and cannot be neatly discharged onto a paper tray.
The aforementioned curling problem of the thermosensitive recording paper is influenced not only by a coating technique for applying a coating liquid for forming a thermosensitive coloring layer on the support sheet, but also by the quality of the support sheet itself. In Japanese Laid-Open patent application 62-23778, in order to reduce the curling tendency, a support sheet comprising a mixture of a natural pulp and a synthetic pulp, with a predetermined stiffness determined by the so-called Clark method, is employed, which stiffness is obtained by bending the sheet in the length direction thereof as defined in JIS P-8143. However, a synthetic pulp is more expensive than wood pulp. In addition, the coating liquid for forming the thermosensitive coloring layer on such a support sheet does not uniformly permeate through the support sheet because the synthetic pulp and the natural pulp are mixed in the support sheet. This has adverse effects on the coloring performance of the thermosensitive recording paper. Moreover, the interaction between the stiffness of the support sheet and the curling problem of the thermosensitive recording paper has not yet been clarified.
In Japanese Laid-Open patent application 60-184875, a support sheet of a thermosensitive recording paper comprises a cationic flexibilizer to prevent the curling problem. However, this attempt does not successfully provide a substantial solution to the curling problem.
In Japanese Laid-Open patent application 61-268482, when a support sheet of a thermosensitive recording paper is prepared by a Yankee paper machine equipped with a drying cylinder, the drying cylinder is preset in such a fashion that the drying conditions at the front side of the support sheet in contact with the drying cylinder are almost the same as those at the back side thereof, and the support sheet is caused to curl in the cross direction (CD) toward the side on which a thermosensitive coloring layer is to be formed. The support sheet is intentionally curled in the cross direction to countervail the curling of the recording paper in the length direction thereof.
However, since the back side of the support sheet is not in contact with the drying cylinder during the preparation thereof and has a low surface smoothness, the drying conditions of a coating liquid for the back side cannot properly be controlled. Consequently, the thus obtained thermosensitive recording paper easily tends to curl in the length direction thereof.
In a thermosensitive recording sheet as disclosed in Japanese Laid-Open patent application 61-14993, the internal binding power of the fibers contained in a support sheet and the water absorption of a fiber node therein are specified. At the same time, the support sheet comprises a bleached kraft pulp of broadleaf tree (LBKP) in an amount of 60 wt. % or more. According to the above-mentioned application, the thermosensitive recording paper thus obtained is capable of yielding images with improved dot-reproduction and high image density. This is because the surface smoothness of the recording paper is improved by containing in the support sheet the bleached kraft pulp of broadleaf tree (LBKP) which essentially consists of short fibers. However, the recording sheet lacks stiffness and the stacking property thereof in a paper tray is not improved.
For the preparation of a support sheet of a thermosensitive recording paper, a pulp, mainly comprising wood pulp, is usually employed, of which beating degree is controlled in terms of the Canadian Standard Freeness (C.S.F.), as defined in JIS P 8121. When the beating degree of the pulp is controlled only by the freeness (C.S.F.), the quality of the support sheets thus prepared greatly scatters and the texture of the support sheet becomes rough. This is because the fibrillating rate of the fibers varies depending on the kind of tree for the pulp, and the unbeaten pulp and unevenness in the fibrillation of fibers cannot be detected when the beating degree of the pulp is determined by the Canadian Standard Freeness (C.S.F.). For these reasons, it is not preferable that the support sheet of the thermosensitive recording paper be prepared by the above method.